mm, compaction: fix wrong pfn handling in __reset_isolation_pfn()

Florian and Dave reported [1] a NULL pointer dereference in
__reset_isolation_pfn().  While the exact cause is unclear, staring at
the code revealed two bugs, which might be related.

One bug is that if zone starts in the middle of pageblock, block_page
might correspond to different pfn than block_pfn, and then the
pfn_valid_within() checks will check different pfn's than those accessed
via struct page.  This might result in acessing an unitialized page in
CONFIG_HOLES_IN_ZONE configs.

The other bug is that end_page refers to the first page of next
pageblock and not last page of current pageblock.  The online and valid
check is then wrong and with sections, the while (page < end_page) loop
might wander off actual struct page arrays.

[1] https://lore.kernel.org/linux-xfs/87o8z1fvqu.fsf@mid.deneb.enyo.de/

Link: http://lkml.kernel.org/r/20191008152915.24704-1-vbabka@suse.cz
Fixes: 6b0868c8 ("mm/compaction.c: correct zone boundary handling when resetting pageblock skip hints")
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reported-by: Florian Weimer <fw@deneb.enyo.de>
Reported-by: Dave Chinner <david@fromorbit.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/compaction.c

@@ -270,14 +270,15 @@ __reset_isolation_pfn(struct zone *zone, unsigned long pfn, bool check_source,
 
 	/* Ensure the start of the pageblock or zone is online and valid */
 	block_pfn = pageblock_start_pfn(pfn);
-	block_page = pfn_to_online_page(max(block_pfn, zone->zone_start_pfn));
+	block_pfn = max(block_pfn, zone->zone_start_pfn);
+	block_page = pfn_to_online_page(block_pfn);
 	if (block_page) {
 		page = block_page;
 		pfn = block_pfn;
 	}
 
 	/* Ensure the end of the pageblock or zone is online and valid */
-	block_pfn += pageblock_nr_pages;
+	block_pfn = pageblock_end_pfn(pfn) - 1;
 	block_pfn = min(block_pfn, zone_end_pfn(zone) - 1);
 	end_page = pfn_to_online_page(block_pfn);
 	if (!end_page)
@@ -303,7 +304,7 @@ __reset_isolation_pfn(struct zone *zone, unsigned long pfn, bool check_source,
 
 		page += (1 << PAGE_ALLOC_COSTLY_ORDER);
 		pfn += (1 << PAGE_ALLOC_COSTLY_ORDER);
-	} while (page < end_page);
+	} while (page <= end_page);
 
 	return false;
 }